The unprecedented accident at the Fukushima Daiichi Nuclear Power Plant following the Great East Japan Earthquake on Mar. 11, 2011 still has had serious impacts on agriculture, fisheries, livestock industry and, above all, the lives of residents living in the vicinity. Japan has been confronted with urgent issues not only in the remediation of the nuclear accident but also in the removal of radioactive substances such as iodine (131I), cesium (134Cs, 137Cs) and strontium (90Sr) emitted in the environment by the accident. In particular, many institutes have been working on various approaches to the decontamination of the environment (in particular, water and soil) from cesium 137 (137Cs) that is the major radioactive substance with a long half-life of about 30 years.
An example of the soil decontamination methods is the physical removal of the surface soil that has been contaminated. However, this method entails problematic treatment of the removed surface soil. Thus, an “on-site replacement method” is recently studied in which the contaminated surface soil is replaced by lower earth (see, for example, Non Patent Literature 1). This method attracts attention because of its freedom from concerns about surface soil treatment and its capability of reducing the radiation dose to 1/10 or below. However, the fact that the contaminated soil is left in the earth raises concerns about the possibility of future contaminations of soil and water.
On the other hand, chemical decontamination methods of recovering radioactive substances with adsorbents or the like have been considered. Adsorbents based on a hexacyanoferrate (II) salt (ferrocyanide) which is a Prussian blue analogue are known as conventional radioactive cesium removers. Various techniques have been reported in order to recover efficiently radioactive cesium from highly radioactive waste liquid discharged from nuclear facilities, with examples including a method for enhancing the cesium adsorption properties of insoluble ferrocyanides and a method in which copper hexacyanoferrate (II) is supported on a porous resin (see, for example, Patent Literatures 1 and 2). Further, Prussian blue itself (iron (III) hexacyanoferrate (II) hydrate) has been pharmaceutically approved (RADIOGARDASE®) in Japan, Europe and the United States as a safe drug which may be used for emergency exposure to remove radioactive cesium from the body (see, for example, Non Patent Literature 2).
Further development of decontamination techniques has been carried out focusing on and with the application of the cesium-binding ability of Prussian blue analogues. For example, it has been reported in media such as newspaper that Tokyo Institute of Technology, Research Laboratory for Nuclear Reactors, developed a system for decontaminating highly contaminated water with Prussian blue (see, for example, Non Patent Literature 3).